Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a Stirling engine.
A Stirling cycle engine depends very importantly
on the operation of a thermal regenerator disposed between
the expansion and compression spaces of the closed working
fluid system. Although regeneration has been studied for
~uite a period of time in connection wit:h the operation of
the Stirling engine, its true theoretical basis of operation
is not completely understood. ~owever~ the regenerator is
designed with certain practical operating conditions in mind.
The design of such regenerator assumes that the temperatures
of the working fluid at the inlet to the regenerator matrix
will be at a certain minimum temperature level, such as
80-C. The design further assumes that even though the inlet
temperature to the matrix will cyclically var~ because the
compression-expansion of the heat input is other than iso-
thermal, the assumption is that such variation will be rela~
tively small within the range o +30~C. Similarly, the ;~
temperature at the exit of the matrix, varying as a practical
matter because of inlet variance and because limited coefici-
ents of heat transfer! it is assumed will not vary considerably
and will be within the limits of, for exampler 750 i 50 C.
With these temperature conditions in mind, the designer then
selects a certain desirable heat capacity for the regenerator
at a certain void volume so as to provide a compro~ise between
tolerable fluid friction therethrough, loss in pressure and
optLmum heat transfer characteristics.
The resultant regenerator, as designed with these
considerations in mind ~y the prior art, does not compensate
for the cold working condition from which a Stirling er.gine
must be started. If a significant goal of the Stirling
engina is to be realized, which includes dramatic fuel savings
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over that o prior art eng.ines, the fuel consumed inraislng the temperature o~ the working fluid from a cold
starting condition must be reduced.
Adding additional heat to the expansion space to
decrease the amount of time tha~ it takes to raise the working
~luid medium to a proper operating temperature is not an
adequate solution by itself. This is in part due to the fact
that the blow time which is defined to be the net time for
flow through the dead space of the system between expansion
and compression spaces, including the void volume within the
regenerator, is extremely short when compared to other prior
art engines, such as a gas turbine engine. ~or example, at ..
moderate engine speeds of 1200 revolutions per minute, the
- blow time is 10 times less than that of the permissa~le
minimum in the gas turbine. In fact, in an engine, which is
o~ moderate size adaptable for vehicular use as a prime mover,~
the blow time will be so short that many particles of working ;~
fluid will never pass completely through the matrix of the
regenerator bafore the flow direction is reversed. The : :
20 very short net flow time through the matrix in one direction
is sl~ghtly less than half the complete cycle time~ccordingly,
the conventional heat transfer process which occurs through
; the regenerator is very complex and incomplete, involving ..
repetitive fluid~to-matrix, matrix--to-fluidt fluid-to-~atrix
cycle relationships.
What is needed is a mechanism or method by which
the working fluid of a Stirling cycle engine can be moved
rapidly ~rom a cold starting state to an operating temperature
condition without reliance upon the normal external circuit
or tha normal transfer of heat from the external heating
circuit through the conventional compression-expansion cycle.
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IE the latter were to be the only alternative solu~ion, it
would be hindered by fluid fric-tion within the working cycle
and the need for a larger void volume within the regenerator
to speed up the temperature increase of the matrix. All of
this would work at odds with the desire for ef~icient oper-
ation at high temperature conditions.
In accordance with the present invention, there is
provided a closed fluid working circuit for a regenerative
type Stirling engine having a conventional electrical cir-
cuit and system for starting, the closed fluid workingcircuit having a plurality of chambers subdivided by double-
acting pistons operating therein, the subdivided chambers
being respectively hot and cold and connected in series
whereby a hot chamber is always in communication with
a cold chamber of the next most adjacent cylinder, the inter-
communication between adjacent cylinders containing a ;~
foraminous regenerator matrix and a cooling mechanism, the
improvement comprising: (a) means defining an elec~rical
heating element invested within the regenerator matrix, the
element extending throughout a ~one of the regenerator
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matrix to ef~ect raising the temperature of said regeneratormatrix to a temperature level substantially above a pre-
determined mean operating temperature within a predetermined
period of time, measured from when the element is energized,
and (b~ control means effective to energize the electrical
element upon closing of the starting circuit of the engine
.
and effective to de-energi%e the element when a predetermined
kemperature level is reached in a hot chamber.
By utilizing the improvement of this invention, the
prior art prohlems are overcome and the Stirling cycle
engine has mor2 responsive thermodynamic characteristics
with greater efficiency and less fuel consumption. A
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decreased start-up time is achieved.
The invention is described furtherr ~y way oE illus-
tration, with reerence to the accompanying drawings, in
which:
Figure 1 is a schematic illustration of a portion of
a working fluid system of a Stirling cycle engine character
istic of the prior art; and
Figure ~ is an enlarged fragmentary view of a portion
o~ the regenerator-cooling apparatus of the system o~ :
Figure 1.
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1 --~A~ B~ P~6~
2 Turning to Figure 1, there is illus~rated a portion
3 of the closed working fluid system 7 of the Stirling-type
4 engine having the pistons arranged in a double-acting manner.
A plurality of cylinders, two of which are shown here as
6 10 and 11, have the volume therein each respectively sub-
7 divided by pistons or reciprocating heads 8 and 9 so that
8 each cylinder will have the variable volume therein comprised
9 of a high temperature (hot) space and a low temperature (cold)
space~ The hot space acts as an expansion volume and the
11 cold space acts as a compression volume. For example, with
12 respect to cylinder 10, the hot space is identified as 13
13 and the low temperature space as 14; with respect to cylinder
14 11, the hot space is identified as 15 and the low temperature
space as 160 Each hot space of one cylinder is connected by
}6 a ~uitable com~unicating means 26 to a low temperature space
17 16 of the next most adjacent cylinder. Such communicating
18 means comprises a gas passage 27 in which is interposed a
19 regenerator 28 and a cooling apparatus ?9, each functioning
in a typical manner in the Stirling cycle engine, where~y gas
21 is bPing displaced from the hot chamber 13 and conveyed through
22 passage 27 allowing the heat content thereof to be absorbed
23 by the regenerator 28 and to be further cooled by mechanism
: 24 29 before entering the low temperature space 16. Such gases
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:25 are agai~ displaced during another phase of the Stirling
26 cycle, from ~he low temperature space 16 back through the
27 passage 27, absorbing heat units from the regenerator 28
28 and again re entering hot chamber 13.
: 29 In practical application, all gas units may not
actually undergo a complete translation from the hot to the cold
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1 chambers but rather there is thermal conductivity that takes
2 place through some of the gas m~dium that is directed along
3 such path.
4 The control and operation o~ a double-acting hot gas
type of engine is more typically described in U.S. Patent
6 3,859,972 which demonstrates a control whereby a change in
7 the mean cycle working pressure will increase or decrease
8 engine speed and torque. Pistons 8 and 9 are mechanically
9 linked with respect to each other in accordance with the
desired timing for variance in the respective space volumes
11 such that piston 8 also extracts work energy during the
12 upstroke for contraction of space 13. When both sides of the
13 sama piston~are utilized for the purposes of serving two
14 separate thermodynamic cycles, the pressures on opposite
i5 sides should be phased to permit the pistons to operate properly.
16 The regenerator matrix absorbs heat units from a
high temperature medium and releases said heat units to a
18 low temperature medium. A typical material u~eful for such
19 matrix comprises a stainless steel wire 30 entrained within
a stainless container 31 and inserted in h~at co~ductive
21 relationship with the ~low pas~ages. Wire diameter is
22 controlled and may be a~ small as .001 inch. Non-metallic
23 regenerator matrices, such as those composed of ceramic
24 material, can also be considered for application of this
concept~
26 The most typical con~iguration for ~uch regen
;;~ 27 erator matri~ is a block having one end 32 adapted to act a~
~28 an inlet for hot gases exposed thereto and an opposite end 33
29 adapted to act as an exit and as a communication with the
cooling apparatus 29. The porosity or void volume within
31 said matrix is designed to provide a propex gas flow
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1 communication during the working cycles of said engine.
2 The void volume should be such to minimize ~riction losses
3 and maximize heat transfer between the matrix and the working
4 gas.
Alternatively, the regenerator can be comprised of
6 a series of woven wire screens sintered together to form a
7' stable semi-rigid block. One mode of m,anufacture is to pack
8 the screens in a desired form and load the form with a weight.
9 The wire screens are then cleansed by nitric or h~drochloric
acid; the loaded assembly is heated for a short period in a
11 furnace with a reducing atmosphere. Upon removal it will be
1~ found that the screens will be sintered into a solid assembly
13 that can be lightly machined. It is important to arrange
14 the screens or the wires normal to the axis of ~low communi-
cation.
16 In all of these constructions of the regenerator,
17~ an independen~ly energized heating element 35 is invested
18 within said matrix and located particularly within the
19 central zone 36 o~ said matrix. The heating element 35 may
be comprised o~ common elactrical wire; it is electrically
21 i~sulated by sheathing 37 to maintain separation between the
22 ~ metallic elements or container 31 of said regenerator and the
23 electrical conductive material of the heating element 35~
24 A control 38 for said heating element is comprised
of a device by which the matrix tempexature can be sensed such
~` ~ 26 that ~hen a preset bulk temperature level is reached, the
27~ auxiliary heating can be switched of~ and the engine continued
28 or restarted in the normal fashion; said control, of course,
29 ~ energizing said heating element upon closing of the starting
30 ~ cirCuit of the engine.
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1 A method by which said matrix can be invested with
2 the heating element is as ~ollows:
3 (a) In the case of a regenerator ~abricated from
4 loose cut wire pieces, the heating element can be implanted
in the container 31 before filling with the wire pieces.
6 When the filling is completed the entirle mass may be sintered;
7 (b) For a regenerator abricated from stacked wire
8 screens, the container 31 can be divided into two portions,
9 each filled in a normal manner with the wire screens. The
heating element can then be inserted be~ween the two completed
11 portions of the regenerator, and the entire assembly brazed/
12 sintered together; and
13 (c) For a matrix fabricated from a non-metallic
14 ox ceramic material, the heating element can be installed in
a manner similar to (b) above.
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